Blue- and red-shifts of V₂O₅ phonons in NH₃ environment by in situ Raman spectroscopy

A layer of ∼30 nm V₂O₅/100 nm-SiO₂ on Si was employed in the in situ Raman spectroscopy in the presence of NH₃ effluent from a thermal decomposition of ammonium acetate salt with the salt heated at 100 °C. When the layer is placed at 25 °C, we observe a reversible red-shift of 194 cm^-1 V₂O₅ phonon by 2 cm^-1 upon NH₃ gas injection to saturation, as well as a reversible blue-shift of the 996 cm^-1 by 4 cm^-1 upon NH₃ injection. However when the sensing layer is placed at 100 °C, the 194 cm^-1 remains un-shifted while the 996 cm^-1 phonon is red-shifted. There is a decrease/increase in intensity of the 145 cm^-1 phonon at 25 °C/100 °C when NH₃ interacts with V₂O₅ surface. Using the traditional and quantitative gas sensor tester system, we find that the V₂O₅ sensor at 25 °C responds faster than at 100 °C up to 20 ppm of NH₃ beyond which it responds faster at 100 °C than at 25 °C. Overall rankings of the NH₃ gas sensing features between the two techniques showed that the in situ Raman spectroscopy is faster in response compared with the traditional chemi-resistive tester. Hooke's law, phonon confinement in ∼51 nm globular particles with ∼20 nm pore size and physisorption/chemisorption principles have been employed in the explanation of the data presented.